a. Field of The Invention
The present invention relates to an endoscope (not flexible) comprising an image-transmitting optical system which transmits an image in turn by a plural number of relay lenses.
B. Description of the Prior Art
An image-transmitting optical system used in endoscopes generally comprises, as shown in FIG. 1, field lenses 1 and imaging lenses 2 which are positioned alternately in an outer tube 4 with pre-determined airspaces by using tubular spacers 3 and is arranged to transmit an image in turn by means of said field lenses and imaging lenses. The intensity of transmitted light L transmitted by the above-mentioned image-transmitting optical system of known endoscopes is expressed by the following formula when reference symbol N represents the numerical aperture of the optical system and reference symbol A represents the area of transmitted image. EQU L .varies. N.sup.2 .times. A
when the focal length of the optical system is represented by reference symbol f and the effective diameter of the imaging lens 2 is represented by reference symbol a.sub.2, the numerical aperture N isexpressed by the following formula. ##EQU1## On the other hand, the area of transmitted image A is expressed by the following formula when the effective diameter of the field lens is represented by reference symbol a.sub.1. ##EQU2## Therefore, the intensity of transmitted light L is expressed as follows. ##EQU3## For the above-mentioned image-transmitting optical system for known endoscopes, the intensity of transmitted light L.sub.0 becomes as follows when the inner diameter of the tubular spacer is represented by reference symbol a, ##EQU4## BECAUSE, EQU A.sub.1 = A.sub.2 = A AND N = L
As described in the above, for the image-transmitting optical system for known endoscopes, the intensity of transmitted light is limited by the focal length of the optical system and inner diameter of the tubular spacer and, therefore, it is difficult to obtain sufficient intensity of light.
If, however, it is possible to position respective lenses in such image-transmitting optical system for endoscopes without using the tubular spacers 3, effective diameters of both the field lenses 1 and imaging lenses 2 respectively become equal to outer diameters of respective lenses, and the intensity of transmitted light L in that case can be expressed by the following formula. ##EQU5##
In the above, reference symbol m represents the ratio between the inner diameter a of the tubular spacer and outer diameter b of the lenses, said ratio being ##EQU6##
It is, therefore, possible to increase the intensity of transmitted light to m.sup.4 times compared with known endoscopes. When, for example, the inner diameter a of the tubular spacers is 2.2 and the outer diameter b of lenses is 2.5, it becomes ##EQU7## and intensity of transmitted light becomes 1.668 times.